Backward wave tube



June 16, 1959 H. HEFFNER ETAL 2,891,191

' BACKWARD WAVE: TUBE Filed Nov. 18, 1953 2 Sheets-Sheet 1 RR .53 l IIE FF mwb FDI EM Y Hm HR.

SEI

June 16, 1959 H. HEFFNER ET AL BACKWARD WAVE TUBE Filed Nov. 18, 1953 2 Sheets-Sheet 2 gl. HEFFNR 1 l a l 1 l l /A/I/ENTORS! United States Patent wur,TUBE` Hubert Hefuer, Summit, and Rudolf Kompfnen Far NJK, assignorsto' Bell Telephone Laboratories, Incorporated, New York, N.Y.,a` corporation of New Yuriy Arplietion, lStrye111leir 18, 195.3, .Serial Na@92,946A .l

11 claims. (ci. sis- 3.6)

This invention relates to electronic apparatus of the kind "now generally designated as traveling Wave tubes which utilizekthe interaction between atraveling 'electromag'netic wave and an electron stream.

An importantclass of such tubes utilizes the interaction between an electronstream Yand an oppositely directed or backward traveling wave, For operation inf this way, thereis employed an interaction circuit for a propagating the traveling wave which has the characteristic that a wave traveling 'with the group veloeity therealong in a given direction will giverise tuy componentsy which have a phasevelocity in the `apposite direction. (It will be' convenient to term such an`interaction cir-U cuit as a backward wave circuit.)` i The electron beam.`

in turnis adjusted 't'o havea velocitysubstantially equal l" to the'phase veloeity "ofhone"y of such componentspwhereby cumulative interaction' between fthe"` beam and the` ywave component is made" possible and amplification of"A a,

the waveisachieved.

Operation in this Waymhas two primary applications. Most importantly, operation in this wayhas `formed the basis of backward`wave oscillators.'

11i the usualformf of such oscillators,` an electron stream is projected past a dispersive backward Wavekcircuit'which is made Asub-` stantially reectionless at itsl dovvnstreani end` and `from whicloscillatory energy abstracted `at the upstream endl In 'this application," the terms fupstrearn and.` downstream are-used with referenceto 'the` ,relativesv separa,-

tion` along 'the path fofiiow from the, electron. source,` n

upstreani indicating; proximity and p downstream" "remote- Thus the directionN of pointsof higher levelto` points of lower. level along path" of iiowV `for "sustaining Aoseillations. 4 advantage of suchobackwardffwave oscillators is that the over a Iwide"continuous band ef -frequencieshis Vproperty` frequency ,y moduf y s uchf oscillators wellfsuited as An importantl s frequency fof 4the.osc'illationsf'can be tuned electronically,.-

latorsl However,4 the full advantage of Athis` property l of backward wave oscillators hasnot readily-beenrealizable hitherto `because `of the `d iiiiculty in abstracting the` oscillatory energy from` thebackward waveHcirciLeiii-J ciently and'withorut reflections lovera wideband of fref quencies. This has been so to a. considerable ,degree because the nature of eiiicient backward wave circuitsis such that broad band coupling connectionsy thereto are diiiicult and awkwark.

` Additicnally, "the problerir` is complicated because the impedancelof the mostfel'licientnf hl` backward lwaive .circuits oscillation band, `makir`igit diiicnlt` to' wellmatch'ed t the circuit` continuously for good coupling.

Accordingly, one object of the present invention is to as is important ICC 2` facilitate the enicient abstractionlof oscillatory energy from a backward wave oscillator.l

Another object is to increaseI the frequency bandt over which a backwardmwave oscillator may be convenieutly and efficiently operated.

Antherobject is to make practical in traveling wave tubes the use of backward wave" circuits which although of high impedanlceland thus well suited `for propagating a waves for" interaction with an electron beamrcannot readily beA coupled to `tl'iemoreuusual forms of Wave transmission lines.

Another object `is to makethe output level. of a backward wave oscillator more uniform with frequency overy its operating range.

To these ends, a backward wave oscillator in accordance with the invention is `modified to include in addii tion to its wide range -`backward wave circuit a separate broad brand forward wave typeinteraction circuit, for

exa`n`1ple,ar heli4rto Awhich an eiiicient broad band Y, coupling connectioncanbe made readily and which is t positioned downtsreatnrelative to the backward wave circuit. `In such anoscillator, `the electron stream along which have been*established` modulations in the formrof n electron bunches lasa result Vof backward Wave interaction is permitted `toltravel pastthe forward wave type cir,- cuit on which Ythe electron bunchesQ induce a forward traveling wave with whichn they in turn interact. r This t forwardwave is thenabstrarcted at the downstream end of thetorjward"*wave circuit. The inclusion of thefor- Ward wave circuit lin .this fashion `serves first -to provide a broad band means for converting the oscillatory n space `charge waveshetvup" onthecelectron beam in the formof electronbunches .into an electromagnetic wave.` Additionally, it serves to amplify thepspace, charge wave on `the streams'and permits "the eventual, abstraction of a higher levellsignal .than'could haverbeen abstracted f originally in the region immediately pastthe region` of, n

backward wave interaction. 1

This latter characteristic oiersfpurther important advantages. It makes possible the realization of oscillatory waves of high level with shortregions of backward wave interaction. .y Moreover, since the speed `with which the; n oscillatory frequency of a backward Wave oscillatorcan lbe varied for frequency modulation applications is related to the length4 of this backwardA ywave region, it is usually desirable for such applications to have `the backward Wave interaction region as short as possible. Accordingly,` an oscillator lator.

Additionally, an oscillator in accordance with the invention can be made to have a more nearly constant power output over its operating band than previously a known backwardY wave oscillators by making internally p both ends of the"` ybackward` wave circuit substantially s reliectionless, and operating the` forward wave circuit at a constant voltaget Alternativelyfamplitude modu-.r

lation may be -impressedon the Voscillatory wave by A varying in accordance with modulating intelligence either the voltage on current in theelectron. beam. i

Additionally, theroscillatora can be modified to mix a radlo frequency signal with the oscillatory wave by applying such a signal as an input to the upstream end of thejgforward wave circuit `for interaction with `the electron beam on which already exists an oscillatoryspacew charge wave. l

The other important application of backward `wave In the usualv input waves `to be amplified are applied to therdcwnstreamendtof a backward wave circuit i interaction is, in a` backward wave amplier. form `of such an amplifier,

Patented Jumel, 1959 a in accordance with Athe invention will 4be well suited for use ,as a frequency modu- `the Aforward wave circuit aroundthe value which results in maximum amplification or thel and the amplified output wave is abstracted at the upstream end of the circuit. An important advantage of such an amplifier is that while the frequency band over which amplication may be secured for a given electron beam velocity is small thispass band can be readily shifted over a wide region merely by varying the velocity of the electron beam. Accordingly, such an amplifier is` well suited for use as an electronically tunable filter. However, one of the problems in the use of such an amplifier is the strong tendency it has to oscillate in a backward wave mode when high gains are sought, which tendency restricts the gain which can be achieved stably.

Accordingly, another object of the present invention is |to increase the stability of backward wave amplifiers and so make possible increased amplification.

.To this end, a backward wave amplifier in accordance with the invention includes a separate forward wave circuit along they path of ofw positioned downstream Vrelative to the backward wave interaction region. In operation, input waves to be amplified are applied to the downstream end of the backward wave circuit for travel therealong for impressing a relatively low level space charge wave on the electron stream which, in turn, induces a forward traveling Wave in the forward wave circuit which is further amplified and then abstracted at the downstream end of the forward wave circuit. Such an amplifier retains the useful electronically tunable filter properties of thc backward wave amplifiers known hitherto and yet permits high and relatively uniform stable gain in the pass band. Stability is enhanced by utilizing the backward wave interaction circuit to excite a space charge wave on the electron beam which although of sufficient level to induce a forward traveling wave on the forward wave circuit is of insufficient level to initiate and sustain backward wave oscillations.

Moreover, in the course of illustration of the principles of the invention, several novel forms of backward wave circuits will be described.

The invention will be better understood from the following more detailed description taken with the accompanying drawings in which: l

Fig. 1 shows a frequency modulator in accordance with the invention which utilizes a bifilar helix as the backward wave circuit;

Fig. 2 shows Van amplitude modulator inaccordance with the invention which utilizes a loaded Lecher-line as the backward Wave circuit; Y

Figs. 3A through 3D show a radio frequency mixer in accordance with the invention which utilizes a loaded wave guide as the interaction circuit; and

Figs. 4A and 4B show a backward wave amplifier which employs a slotted hollow cylindrical conductor as the backward wave circuit.

Referring now more particularly to the drawings, in the tube shown in Fig. l the various tube elements are enclosed in an evacuated glass envelope 11. At opposite ends of the envelope are positioned an electron source which is the cathode 12A of an electron gun 12 and the collectorv electrode 13 positioned in target rela* tion with the source. The electron flow is focused by suitable means, not shown here, to be coaxial with the axis of the tube envelope. Positioned along the upstream portion of the path of flow is the backward wave circuit comprising a bifilar helix 14 which serves effectively as a helically coiled balanced transmission line. 'I'he general principles of the use of a biflar helix as a backward wave circuit are described in detail in U.S. Patent 2,843,792, issued July 15, 1958, of J. R. Pierce. Positioned downstream along the path of flow is the forward .wave circuit comprising the single wire helix 15. The helix 15 is positioned coaxial with the tube envelope and with the path of flow. .The bifilar helix 14 on the other hand is positioned vto have itsV axis parallel to but displaced from the tube axis and the axis of the path of fiow. The reasons for this will be discussed hereinafter.

In the backward wave oscillators known hitherto, it is the practice to terminate the downstream end of the backward wave circuit to make it substantially refiectionless and to abstract the oscillatory wave at the upstream end of the circuit by a suitable external coupling connection. In the present tube, both ends of the backward Wave circuit are terminated internally to be substantially reflectionless. For this purpose, for example, dissipative material, such as aquadag, is coated along the inner surface of the envelope along portions adjacent the end turns of the bifilar `helix, the thicknesses of the end coatings 16 and 17 being tapered to provide a broad band dssipative termination. Additionally,the upstream end of the forward wave circuit is terminated in a similar manner by the coating 18 deposited along the inner surface of the envelope along portions adjacent its upstream end turns. The useful energy is abstracted at the downstream end of the helix 15 by suitable coupling means. In the coupling means illustrated, the downstream end of the helix 15 is connected by way of the impedance matching section 19 to the coupling strip 20. The impedance matching section 19 comprises a section of helix in which the helix pitch increases gradually. The coupling strip 20 is positioned to couple to the electric vector in the rectangular wave guide 2,1 which is closed at one end and leads to the utilization means at its other end. Various other arrangements are known to workers in the art for coupling an external wave transmission line to a single Wire helix. Although the interaction of the beam and the wave on the forward wave circuit results in forward Wave type amplification, in general, it should be unnecessary to distribute dissipative material along the intermediate portion lof the forward wave circuit to minimize the tendency to self-oscillation as is characteristic in the conventional forward wave traveling wave amplifier, inasmuch as the internal termination of the helix should result in little reflection from the upstream end of the forward wave circuit. However, in the case that reections are unavoidable, any of the various arrangements known to the art for avoiding self-oscillation may be employed.

In operation as a backward wave oscillator, the velocity of the electron beam past the backward wave circuit is adjusted to be in synchronism with the phase velocity of a spatial harmonic component of a wave of the desired frequency setfupby noise components on the beam at the downstream end of the backward wave circuit for travel upstream therealong. As is set forth in the Pierce patent, in a tube which uses a bifilar helix as the backward wave circuit the velocity of the electrons should be adjusted to be approximately maintained by lead-in conductors from voltage source 22. In some arrangements, it will be convenient to maintain a D.C. voltage difference between the two conductors forming the bilar helix for electrostatic focusing, but in the interest of simplicity the two conductors of the bifilar helix arehere shown at the same positive D.C. potential with respect to the electron source as is the case where magnetic focusing of the 'beam is employed.

For use as a frequency modulator, the modulating intelligence is used to vary the velocity of the electron beam past the backward wave circuit, thereby changing I... s? correspondingly the oscillatory frequency. To this end,

a -source 23 of modulating voltage-under `control of the', modulating `intelligence is inserted seriallyin the conductive path between theuelectron` source and the backward wavecircuit. .M

`In operation, the backward traveling growing oscillatory wave bunches the forward flowing electron beam which may beconsidered as giving rise thereon to a forward traveling space charge wave. This space charge wave subsequently inrits passager` past the forward wave circuit induces a forward-traveling electromagnetic wave in the forward wave circuit whichutravels therealong.` By properly choosing the retardation characteristics of this forward wave circuit, this `travelingwave` can be made to interact with` the electron t. beam whereby it is` amplified. T he` amplified wave is `abstracted for utilization at the downstream end of the forward wave circuit by the output coupling means. For interaction, the velocity ofthe electron beam past the forward wave circuit is adjusted to be approximately equal to the phase velocity of the forward traveling wave' alongthe helix 15 in accordance with the usual practice in helix-type traveling wave tubes. different from the velocity of the beam past the backward wave circuit. The relative velocitiespast the two circuits are dependent on the relative voltages on the two circuits. For controlling the electron velocitya potential difference is maintained between the electron source and the helix V by suitable lead-in conductors fromxthevoltage source 24. Because of the broadband characteristics `of a single wire helix as a forward wave interaction circuit, good amplification of the forward traveling wave induced by the space charge oscillatory waves can be achieved over the wide band of frequencies over-Which theoscillatory wave can be modulated. Y i i It-isimportant to displace the axis of the bifilar helix relative tothe axis of thesingle `wire helixtosecure electron beam bunching by the bilar helix of akind which is bestadapted to induce a forwardtraveling wave on the single wire helix. Because of spatial harmonic components `of a wave traveling along the` bilar helix are quite asentar surrounding the path of electron flow. A wave traveling along the parallel conductors will because of Athe presence of the cylinders 46 setup strong spatial harmonic components along with the tube axis which can be made spatial harmonic components may be `adjusted by the periodicity and length of the cylinders. The velocity fof the electron beam may be adjusted by the accelerating voltage provided between the succession of annular cylinders and the electron source.

ence is maintained yby lead-in conductors from the voltage supply'47. The frequency of the oscillatory wave is This velocity can .either be equal to or small along the helix axis, for `good spatial harmonic f interaction it is advantageous to displacethe path of flow ofthe beam from-the helix axis to a regionof` stronger spatial harmonic components. However; in the case of the single `wire helix, interaction with ther fundamental mode ofthe traveling wave which is` strong along the helix axis is usually preferable `to spatial harmonic interaction, and, accordingly,jitris desirable to havethe path of electron flow past the single wire helix along the helixaxis.

To improve the uniformity of the output `level with changes in frequency, the forward wave circuit can be` operated to have a frequency response which is complementary to the frequency response to the backward wave circuit. Alternatively, the frequency responses of` thebackwardand forward wave circuits may be `chosen to provide an output of anydesiredfrequency `characteristic. i

ln the amplitude modulator et) shown in Fig. Z, the various tube elements are housed in a glass envelope 41. An electron gun 42 including a cathode 42A and a collector 43 define a path of longitudinal electron flow coaxial with the longitudinal tube axis. i Suitable` means not shown are employed to maintain the electron beam aligned with the `tube axis. Such means may include either magnetic focusing equipment ortmeans for achieving a spatially alternating time-constant field` along the path of` flow. The backward wave circuit utilized to modulate the electron beam and set up space charge oscillatory waves thereon comprises a Lecher system including the two parallel straight conductors 44 and 45 positioned diametrically opposite along the tube axis alternately from` which` is conductively `supported a linear array of annular conductive Ycylinders 4,6` which are spaced varied to tune the oscillatory frequency.` To make possible oscillationsof relatively uniform amplitude over a wide range, the two ends of the backward wave circuit are made substantially retlectionless by the insertion in- `ternally of dissipative terminations.

To this end, at each end of the backward wave circuit there is inserted a pair of dielectric rods 48, 49, each of which rods is coated with lossy material, the thickness of the coating `increasing gradually to make reflectionless `terminations of each pair of rods.

alongthe backward wave circuit, but in the interest of simplicity `inthe drawing they are shown of abbreviated length. The two rods of each pair preferably are disi posed diametrically opposite one another and adjacent to the glass envelope 4l in the regions between conduc-` tors Maud 45.

The forwardwave circuit comprising the single wire helix 5t) is positioned along the path of flow coaxial with the tubefaxis downstream of the forward wave circuit "waves set up on theelectron beam by the backward wave circuit induce a forward traveling electromagnetic wave on the helix.

whereby the oscillatory spaced charge The helix 5@ is made substantially reilectonless at its upstream end by a dissipative coating 5l alongtheinner surfaceof that portion of the tube envelope adjacent to its end turnsas described above. The output wave is abstracted at thefdownstream end of the helix by a suitable coupling means 52 which, for example, is of the kind described above in the tube shown in Fig.` l.

` The retardation characteristics ofthe helix are in this case also chosen so that the phase velocity of the fundamental component of a forward traveling wave is approximately equal to the velocity of the therealong beam therepast, which beam velocity is controlled by the voltage diiference'between the electron source and thehelix. However, it is a well-known characteristic of helix-type traveling wave tubes that there exists an optimum `beam voltage, dependent on various parameters, `forwhich there results maximum interaction between the 52 of `modulating voltage is inserted in series with the` `voltage supply 53 to establish by-suitable lead-in conductors the accelerating potentials on the helix 50.

It can be seen that by the principles of the invention there can becombined in `a single tube both the oscillatory and amplitude modulating functions in "a cooperaf tive manner which results in no mutual interference.

Alternatively, amplitude modulation may be achieved by modulating in accordance with signal intelligence the current of the electron beam, `provided only that precautions are `taken to insure that the current never falls belowy A suitable potential diier-` The rods extend l, preferably for a. distance of several operating wavelengths the tube shown in Fig.` 2, intelligence to vary the the amount needed to sustain oscillations on the backward wave circuit.

VIn the radio frequency mixer 60 shown in Figs. 3A through 3D, the various tube elements are enclosed in an evacuated glass envelope 61. At opposite ends, an electron source 62 and a target electrode 63V define a path of electron flow coaxial with the longitudinal axis of the tube. The backward wave circuit in this tube comprises a conductive cylindrical wave guide 64 which extends along an upstream portion of the path of flow and which is loaded by a succession of transverse slat members 65 which extend across the wave guide in a di-ametric fashion. The slat members are spaced apart along the path of flow, and each successive member is rotated about the longitudinal axis a predetermined angle with reference to the immediately preceding member. Each of the slat members is apertured and the apertures are aligned in a linear array coaxial with the tube axis for passage of the electron flow therethrough. Moreover, from each slat there is supported a short vconductive cylinder 66 for surrounding the path of flow. A voltage source 73 is connected between the Wave guide 64v and the electron source 62. 'It is found characteristic of such a wave circuit that a Wave propagating in a direction opposite to that of electron iow will set up along the tube axis stron-g spatial harmonic components. In particular, it is characteristic that the fundamental or lowest order spatial harmonic mode of a wave whose group velocity is in a direction opposite to that of electron flow will have a phase velocity in the direction of electron flow. A circuit of this kind is analogous to an artificial line made up of a ladder network of shunt inductances and series capacitances, the successive slat members acting as inductive posts for forming a series of shunt inductances and the spaced array of cylinders serving to introduce the intermediate series capacitances. In such an artificial line, the phase shift advances in a direction opposite to the direction of wave propagation along the line.

To terminate internally the two ends of the backward wave circuit, there are inserted pairs of dielectric rods 67 and 68 coated with dissipative material which resemble those inserted in the tube shown in Fig. 2.

The forward wave circuit which again comprises a single wire helix 69 is disposed along the path of flow downstream of the backward wave circuit for utilizing the oscillatory space charge waves set up on the beam in the manner described above. For operation as a radio frequency mixer, an input wave which is to be mixed with the oscillatory wave is inserted into the forward Wave circuit at its upstream end by coupling means 70, and the mixed signal is available at the downstream end of rthe forward wave circuit for abstraction by output coupling means 71. In this instance, it is desirable to distribute dissipative material in a coating 72 along the forward wave circuit to minimize the tendency towards self oscillation of this circuit resulting from wave energy reflections at the input and output connections to this circuit. Alternatively, the tendency to self oscillations may be ourbed by making this helix'substantially nonreciprocal in its attenuation properties by the insertion therealong of a magnetically biased ferrite element, as is described in copending application Ser. No. 362,177, filed June 17, 1953, by R. Kompfner and H. Suhl. For accelerating the electron ow past the forward wave circuit, the helix is maintained at a suitable positive potential with respect to the electron source by lead-in conductors connected to the voltage supply 74. The retardation characteristics of the helix and the velocity of beam are-adjusted for interaction between the beam and the Iwave.

In operation, the oscillatory space charge wave on the electron beam sets up avforward traveling wave in the forward wave circuit which is mixed with the input Wave also applied thereto. Then when the levels of the two signals become suiciently high to result in operation in the nonlinear region of the tubes amplifying characteristics, there result the modulation products of the two waves of which a particular one may be favored by operation in a dispersive region of the circuits phase velocity versus frequency characteristic. Additionally, it may be desirable to lter the output to remove the fundamental frequencies and undesired modulation product frequencies.

With reference now to Figs. 4A and 4B, the various elements of the backward wave amplifier are enclosed in the evacuated glass envelope 81. At opposite ends an electron source 82 and a target electrode 83 define a path of ow of an electron beam along the longitudinal axis of the tube. Suitable focusing means not shown are utilized to keep the electron ow straight. upstream along the path of electron ow is the backward wave circuit which in this instance comprises the annular conductive cylinder 84 which has a succession of lateral yslots or cuts 85 spaced apart longitudinally therealong. The slots are cut in an interleaved pattern, adjacent slots being on diametrically opposite sides of the cylindrical surface. The slots are relatively deep, leaving only relatively small conductive strips 86 integral with the conductive surface in the regions of the slots. Such a structure exhibits along its axis strong spatial harmonic components of a wave propagating therethrough. Accordingly, the cylinder 84 is positioned to have its axis parallel to the longitudinal axis of the tube along which ows the electron beam. An input wave guide 88 of rectangular cross section is used to supply input signals to the downstream end of the backward wave circuit. To this end, the wave guide 88 has its narrow side walls apertured for passage therethrough of the tube envelope and the wave guide is positioned along the tube envelope so that the vdownstream end of the cylinder is bridged across the apertured `side walls of the guide. Toward the downstream portion of the cylinder the depth of the slots decreases gradually forming a tapered transition region 89 to which the wave guide is coupled. Additionally, a conductive sleeve 90 extends from the wave guide wall for shielding that portion of the transition region not included within the wave guide. One end of the wave guide is closed and the other end leads olf to a signal lsource. The upstream end of the backward wave circuit is made substantially reectionless by the insertion of a dissipative termination 87 comprising a pair of coated dielectric rods of the kind shown for terminating the backward wave circuit shown in Figs. 2 and 3. In this case the rods are positioned intermediate the glass envelope 81 and the conductive cylinder 84, as shown in Fig. 4B.

The forward wave circuit again comprises a helically wound conductor 91 disposed downstream along the path of ow coaxial with the tube axis whereby the electron beam passes axially therethrough. The upstream end of the forward wave circuit is made substantially reectionless by the insertion of a resistive termination in the form of a coating 92 of dissipative material on the portion of the inner surface of the envelope adjacent the upstream end turns of the helix 91. It should not be necessary to insert loss along the intermediate portion of the forward wave circuit for tube stability if the upstream end of the forward wave circuit is made sufficiently reiiectionless. The amplified output wave is abstracted at the downstream end of the forward wave circuit by coupling means 93.

In backward wave amplification operation, it is important to make the backward wave circuit suciently short and the electron beam 'current therepast suiciently low that oscillations in the backward wave mode are not initiated. The backward wave circuit portion of the tube serves primarily as a narrow pass band filter whose pass band can be varied electronically. The pass band is controlled by the velocity of the electron beam past the backward wave circuit which can be varied by chang- Positioned ing the voltage difference wave is suiciently near to the velocity ofthe electron beam whereby good interaction between the beam,` and wave results. `'Ille velocity ofthe beam is controlled by the voltage supply` 95 used to `establish a potential difference between the helix and the electron source.

As has been indicated above, inbackward wave ampliers it is important that the backward wave gain be insucient to initiate oscillations in a .backward wave mode, This has proved one of the limitations which have made it difcult to exploit fully thetdesirable properties of such ampliers. In the embodiment described this problem is avoided by making the backward wave circuit no` longer than necessary `to bunch the electron beam in accordance with the `input signal, `and deriving amplication primarily from the action ofthe forward wave circuit. However, this embodiment still retains the advantages of a conventional backward wave amplifier in that the electron bunching process will be controlled by the velocity of the electron beam, the velocity of the beam having to be such as to permit interaction between the beam and a forward traveling spatial harmonic of the backward traveling signal wave. This makespossible electronic control of the pass band properties of the am plifier. Additionally, where high output powers are desired measures maybe taken to increase the beam current past the forward wave circuit as, for example, by the provision `of an additional electrn source` in `the region intermediate the backward and forward wave circuits.

It should be evident from the that `the various preceding description 'specific embodiments illustrated are provided between the electron source S2 `and the conductvecylinder M854., This voltage,

merely illustrative `of the general principles of the invention. Various modications maybe devised by one skilled in the art without departing fromthe' spirit and n scope of the invention. both backward and forward wave interaction circuits may be employed. Additionally, the path of flow of the electron beam need riotmbcstraight, but may be circular as is characteristic of someforms of traveling wave tubes, particularly those" `of "the magnetrontype which employ crossed electric and magnetic fields. Additionally, pulsed operation maybe achieved by the insertion of suitable detlectingmeans interposed between the back-` ward and forward wave circuits for makingthe flow past the forward wave circuit intermittent without disturbing the oscillatory behavior of the backwardwave circuit. Such a modicatinn in tube of Fig. 1,V for example, makes available at the output a train of pulses,

each of which is frequency 'modulated but whose phase is continuous from pulse to pulse.

What is claimed is: n

l. In an electron discharge device in combination, an electron source and a target electrode for defining a path of flow for an electron beam, abackward wave interaction circuit characterized in that a wave traveling with a group velocity therealong `in a given direction will give rise to space harmonic components which have a phasevelocity in the opposite direction positionedupstream along the path of electron flow for yimpressing space charge waves on the electron beam,cdssipative termination means locatedsolely within` said device for making the upstream end of the backward wave circuit` In particular, various forms of substantially retlectionless, ai, forward wave interaction circuitlpositioned .downstream along the `path of ow withrespect to said backward wave circuit in which the..

space Vcharge waves on` the` electron beam induce forward traveling electromagnetic waves, and means to couple output wave energy from the downstream end of. the forward wave circuit. t

2. In an oscillator, an electron source and a target electrode dening` a path of ow for an electron beam a backward wave interaction` circuit characterized in that` a wave traveling with4 a group velocity therealongin a` glven direction will giverise to space. harmonic components which have a phase velocity in the opposite direction positioned upstream along the path of flow for ward wave interaction` circuit positioned ydownstream along the path of` flow with respect to: said backward` wave circuit inwhich the oscillatory space charge waves excite forward traveling waves, Vand means to couple oscillatory wave energy from the downstream end of` the forward wave circuit. c

3. In a frequency modulator, an electron source and `a target electrode` for defining a path of ow for an electron beam, a backward wave circuit characterized in that a wave traveling with a group velocity therealong in a given direction will `give rise to space harmonic components which have a exciting space charge waves on the electron beam, dissipative termination means located solely within saidV modulator for making the upstream end of the backward wave circuit substantially,reilectionless, means controlled by modulating intelligence for varying the` velocity of the electron beam past the backward wave circuit, a forward wave circuit positioneddownstream along the path of ow with respect to said backward wave circuit direction will give rise to, space harmonic components. which have a phase velocity in` the opposite direction positioned upstream along the path of flow for impressing space charge waves on the electron beam, dissipative termination means located solely within said modulator for making the upstream end of the backward wave circuit substantially rellectionless, a` forward wave circuit positioned downstream along the path of flow with respect to said backward wave circuit in which the space charge waves excite forward traveling electromagnetic waves, means under the control of modulating intelligence for varying the velocity of the electron beam past the forward wave circuit, and means for coupling output `wave energy from the downstream end of the forward wave circuit.

5. In a radio frequency mixer, an electron source and a target electrode defining a path of ow for an electron beam, a backward wave circuit characterized in that a wave traveling with a group velocity therealong in a given direction will give rise to space harmonic components which have a phase velocity in the opposite direction positioned upstream along the path of ow for impressing space charge waves on the electron beam,`

respect to said backward wave circuit in which the space i phase velocity in the opposite. direction positioned upstream along the path of ow for` 11 charge waves excite forward traveling electromagnetic waves, means for applying a mixing signal to the upstream end of the forward Wave circuit, and means for abstracting the mixed wave signal at the downstream end of the forward wave circuit.

6. In an electron discharge device in combination, an electron source and a target electrode defining a path of flow for an electronrbeam, a backward wave circuit characterized in that -a wave traveling with a group velocity therealong in a given direction will give rise to space harmonic components which have a phase velocity in the opposite direction positioned upstream along the path of flow for propagating therealong an electromagnetic wave in a direction opposite -to the path of electron flow and inducing a space charge wave on the electron beam which travels in the direction of electron flow, dissipative termination means located solely within said device for making the two ends of the backward wave circuit substantially reflectionless, a forward wave circuit positioned downstream along the path of flow with respect to said backward wave circuit in which the space charge wave excites a wave traveling in the direction of electron flow, dissipative termination means for making the upstream end of the forward wave circuit reflectionless, and means for coupling wave energy from thedownstream end of the forward wave circuit.

7. In a frequency modulator, an electron source and a target electrode defining a path of flow for an electron beam, a backward wave circuit characterized in that a wave traveling with a group velocity therealong in a given direction will give rise to space harmonic components which have a phase velocity in the opposite direction positioned upstream along the path of flow for establishing a forward traveling space charge wave on the electron beam, dissipative termination means located solely within said modulator for making the two ends of the backward wave circuit substantially reffectionless, means under the control of modulating intelligence for varying the velocity of the electron beam past the backi ward wave circuit for varying the frequency of the space charge wave on the electron beam, a forward wave circuit positioned downstream along the path of flow with respect to said backward wave circuit in which the forward traveling space charge wave excites `a forward traveling electromagnetic wave, dissipative termination means for making the upstream end of the forward wave circuit substantially reflectionless, and means for coupling wave energy from thel downstream end of the forward wave circuit. i l

8. In an amplitude modulator, an electron source and a target electrode for defining a path of flow for an electron beam, a backward wave circuit characterized in that a wave traveling with a group velocity therealong in a given direction'will give rise to space harmonic components which have a phase velocity in the opposite direction for inducing a forward traveling oscillatory space charge wave on the electron beam, dissipative termination means located solely within said modulator for making both ends of the backward wave circuit substantially refiectionless, a forward wave circuit positioned downstream along the path of electron flow with respect to said backward wave circuit in which the forward traveling space charge wave excites a yforward traveling electromagnetic wave, means under the control of modulating intelligence forv varying the velocity of the electron beam past the forwardV wave circuit, dissipative termination means for making the upstream end of the forward wave circuit substantially reflectionless, and means for coupling output wave energy from the downstream end of the forward wave circuit.

9. ina backward wave amplifier, an electron Vsource and a target electrode for defining therebetween a path of flow for an electron beam, a backward wave circuit characterized in that a wave traveling with a group velocity therealong in a given direction will give rise to space harmonic components which have a phase velocity in the opposite direction positioned upstream along the path of flow, means for applying an input signal to be amplified to the downstream end` of the backward wave circuit for propagation therealong in a direction opposite to that of electron Vflow for establishing on'the electron beam a forward traveling space charge wave, dissipative termination means located solely within said amplifier for making the upstream end of said backward wave circuit substantially reectionless, a forward wave circuit positioned downstream along the path of flow with respect to said backward wave circuit in which the forward traveling space charge wave on the electron beam induces a forward traveling electromagnetic wave, and means for abstracting the amplified electromagnetic wave at the downstream end of the forward wave circuit.

10. In an electron discharge device in combination, an electron source and a target electrode for defining therebetween a path of flow for an electron beam, a backward wave circuit characterized in that a wave traveling with a group velocity therealong in a given direction will give rise to space harmonic components which have a phase velocity in the opposite direction positioned upstream along the path of flow, means coupled to the downstream end of the backward Wave circuit for inducing a wave to be amplified for propagation therealong in a direction opposite to that of electron liow for establishing a forward traveling space charge wave on the electron beam, dissipative means located solely within said device for making the upstream end of the backward wave circuit substantially reflectionless, a forward wave circuit positioned downstream along the path of flow with respect to said backward wave circuit in which the space charge wave on the beam induces a forward traveling electromagnetic wave, dissipative means for making the upstream end of the forward Wave circuit substantially refiectionless, and means coupled to the downstream end of the forward wave circuit for abstracting u,for utilization the amplified electromagnetic wave.

ll. In combination, an evacuated envelope, an electron source and a target electrode at opposite ends of saidV envelope for defining therein a path of ow for an electron beam, a backward wave circuit characterized in that a'wave traveling'with a group velocity therealong in a given direction will give rise to space harmonic components which have a phase velocity in the opposite direction within said envelope positioned upstream along the path of flow for propagating therealong a backward traveling wave which establishes a forward traveling space' charge wave on the electron beam, dissipative termination means located solely within asid envelope for making the two ends of the backward wave circuit substantially reflectionless, a forward wave circuit within said envelope positioned downstream along the path of ow with respect to said backward wave circuit in which the forward traveling space charge wave induces a forward traveling electromagnetic wave, dissipative termination means within said envelope for making the upstream end of the forward wave circuit substantially reflectionless, and means external to the envelope coupled tothe downstream end of the forward wave circuit for abstracting wave energy for utilization.

l2. In combination, an electron source and a target electrode for defining therebetween a longitudinal path of iiow for an electron beam, a bifilar helix positioned upstream along the path of flow for propagating therealong an electromagnetic wave traveling in the direction opposite to that of electron flow and inducing a forward traveling space charge wave on the electron beam, a single conductor helix positioned downstream along the path'of flow with respect to said backward wave c ircuit in which the space charge wave on the electron beam induces a forward traveling electromagnetic wave, and means at the downstream end of the single conductor 13 helix for abstracting the forward traveling wave for utilization.

13. In an electron discharge device in combination, an electron source and a target electrode for defining a Alongitudinal path of fiow for an electron beam, a Lecher system positioned upstream along the path of ow for propagating therea'long an electromagnetic wave traveling in a direction opposite to that of electron flow Y and inducing a forward traveling space charge wave on the electron beam, dissipative termination means located solely within said device for making the upstream end of the Lecher system substantially reflectionless, said Lecher system comprising a pair of parallel conductors from 'which extend a linear array of annular conductive cylinders through which flows the electron beam, alternate cylinders extending from the same conductor, adjacent cylinders extending from different conductors, a single conductor helix positioned downstream along the path of flow with respect to said backward wave circuit in fwhich the forward traveling space charge wave induces a forward traveling electromagnetic wave, and means coupled to the downstream end of the single conductor helix for abstracting the forward traveling electromagnetic wave for utilization.

14. In an electron discharge device in combination, an electron source and a target electrode for defining therebetween a longitudinal path of flow for an electron beam, an annular conductive cylinder positioned for surrounding an upstream portion of the path of flow for propagating an electromagnetic wave traveling in a direction opposite to that of electron flow and inducing a forward traveling space charge wave on the electron beam, dissipative termination means located solely within said device for making the upstream end of the propagating cylinder substantially refiectionless, a succession of transverse diametric slat members spaced apart longitudinally along the cylinder and apertured for passage of the electron flow, successive members being angularly displaced around the cylindrical axis, a single conductor helix positioned along a downstream portion of the path of electron flow in which the forward traveling space charge wave induces a forward traveling electromagnetic wave, and means coupled to the downstream end of the single conductor helix for abstracting the forward traveling wave for utilization.

l5. In an electron discharge device in combination, an electron source and a target electrode for defining therebetween a longitudinal path of flow for an electron beam, a conductive annular cylinder positioned around the upstream portion of the path of electron flow for propagating an electromagnetic wave traveling in a direction opposite to that of electron flow and inducing a forward traveling space charge wave on the electron beam, dissipative termination means located solely within said device for making the upstream end of the propagating cylinder substantially reflectionless, said cylinder having ysuccessive transverse slots in its cylindrical surface, successi've slots being on opposite sides of the cylindrical surface for forming an interleaved pattern of slots, a single conductor helix positioned around the downstream por- 14 tion of the path of electron flow in which the forward traveling space charge wave induces a forward traveling electromagnetic wave, and means coupled to the downstream end of the single conductor helix for abstracting the forward traveling wave for utilization.

16. In an electron discharge device in combination an electron source and a target electrode for defining a path of flow for an electron beam, a backward wave interaction circuit positioned upstream along the path of electron flow for impressing space charge waves on the electron beam, the backward wave circuit being characterized in that a wave traveling with a group velocity therealong in a given direction will give rise to space harmonic components `which have a phase velocity in the opposite direction and being adapted to provide an interaction between the electron beam and the space harmonic components to cause growth of said traveling wave, dissipative termination means located solely within said device for making the upstream end of the backward wave circuit substantially refiectionless, a forward wave interaction circuit positioned downstream along the path of flow with respect to said backward wave circuit in which the space charge waves on the electron beam induce forward traveling electromagnetic waves, and means to couple output rwave energy from the downstream end of the forward wave circuit.

17. In an electron discharge device in combination an electron source and a target electrode for defining a path of ow for an electron beam, a backward wave interaction circuit positioned upstream along the path of electron flow and in which there is induced by the electron beam an electromagnetic wave which travels therealong in the direction opposite to that of electron flow and gives rise to space harmonic components which have a phase velocity substantially equal to that of the electron flow with which components the electron beam interacts, whereby the induced wave grows and space charge watves are impressed on the electron beam, dissipative termination means located solely within said device for making the upstream end of the backward wave circuit substantially refiectionless, a forward wave interaction circuit positioned downstream along the path of flow with respect to the backward wave circuit in which the space charge waves on the electron beam induce forward traveling electromagnetic waves, and means to couple output wave energy from the downstream end of the forward wave circuit.

References Cited in the file of this patent UNITED STATES PATENTS 2,541,843 Tiley Feb. 13, 1951 2,585,582 Pierce Feb. 15, 1952 2,636,948 Pierce Apr. 28, 1953 2,654,047 Clavier Sept. 29, 1953 2,657,305 Knol et al. Oct. 27, 1953 2,720,610 Kazan Oct. 11, 1955 2,730,647 Pierce Ian. 10, 1956 2,802,136 Lindenblad Aug` 6,| 1957 

